Precipitation hardening type single crystal austenitic steel

ABSTRACT

In order to provide an austenitic single crystal stainless steel having preferable stress corrosion cracking resistance, strength, and irradiation induced embrittlement resistance so as to extend the life of a nuclear reactor core structure, which is used under a high radiation dose environment, a method is employed, which comprises the steps of homogeneously dispersing carbides into a parent phase of the austenitic single crystal stainless steel by a two step solution heat treatment, and subsequently effecting an ageing heat treatment after rapid cooling for precipitating fine carbides. Austenitic single crystal stainless steel having preferable stress corrosion cracking resistance, strength, and irradiation induced embrittlement resistance can be provided, and the life of nuclear reactor core structure, which is used under a high radiation dose environment, can be extended.

BACKGROUND OF THE INVENTION

The present invention relates to novel austenitic stainless steel,especially, to novel structural austenitic stainless steel preferablefor use in a radiation irradiated environment, such as nuclear reactorcores and nuclear fusion reactors, and the usage of the same.

Austenitic stainless steel, especially the stainless steel having a highchromium-nickel composition, has been used as a material for structuralmembers in nuclear reactors, because the stainless steel has a corrosionresistance in a corrosive environment in addition to having preferableproperties as a structural material. However, a member made ofaustenitic steel in light water reactor cores becomes increasinglysensitive to Intergranular Stress Corrosion Cracking (IGSCC) with longtime irradiation in use. For instance, a stainless steel having a solidsolution state, obtained by solution heat treatment, has strongresistance to IGSCC outside the reactor core where no radiation damageis caused. However, the same material loses resistance to the IGSCC whenit is irradiated with high level radiation, especially, radiation atmore than 0.5×10²¹ n/cm² in neutron dose, inside the reactor core. TheIGSCC phenomenon, which is called Irradiation Accelerated StressCorrosion Cracking (IASCC), has currently been deemed as a problemrelating to aged nuclear reactors. As for the metallurgical mechanism ofthe IASCC phenomenon, two mechanisms of this phenomenon have been wellknown, including (1) atomic diffusion (irradiation induced diffusion),accompanied by a migration of voids which are generated by irradiation,causes a reduction in the concentration of chromium, a corrosionresistant element, in the vicinity of grain boundaries, and (2) impurityelements, such as P, S, si, and others, are segregated at grainboundaries, which reduces corrosion resistance at the grain boundaries.

As for a method for resolving the above described problems,JP-A-63-303038 (1988) discloses a method where the amount of compositeelements of austenitic stainless steel, such as N, P, Si, S, C, Mn, Cr,and Ni, are adjusted, and traces of Ti and Nb are added to theaustenitic stainless steel.

On the other hand, as for a method for preventing intergranular stresscorrosion cracking, single crystallizing methods for eliminating grainboundaries, which form a network and are a source of the cracking, havebeen proposed. As for the single crystals, a single crystal steel ofaustenitic (γ) single phase having a crystalline structure of FCCstructure, a single crystal steel of austenitic phase matrix including asmall amount of ferritic (δ) phase having a crystalline structure of BCCstructure, and a single crystal steel of a so-called two-phase stainlesssteel, wherein the (γ) phase is dispersed in a single crystal of αphase, are disclosed in JP-A-3-264651 (1991) and JP-A-62-180038 (1987).

However, the invention disclosed in JP-A-63-303038 (1988), whereinintergranular stress corrosion cracking is prevented by adjustment ofthe composition, can not prevent substantially all of the abovedescribed stress corrosion cracking generated by irradiationacceleration, because the material is polycrystalline stainless steeland its structure contains grain boundaries.

Furthermore, the Proposal disclosed in JP-A-3-264651 (1991) relates to asingle crystal steel of γ single phase including steel with Ti, Nbadded, a γ phase single crystal steel containing a small amount of δphase, both of which steels are strengthened by a solid solution ofalloy elements and have a lower yield strength than that of commerciallyavailable stainless steels, SUS 304 steel and SUS 316 steel. TheProposal disclosed in JP-A-62-180038 (1987) relates to a single crystal,of which the parent phase is composed of a α phase having a BCCstructure. The α phase has been a source of concern as it is conceivedto more readily cause irradiation embrittlement by irradiation damagethan the γ phase.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an austenitic stainlesssteel with a stable structure, a resistance to stress corrosioncracking, a preferable strength, and a resistance to irradiationembrittlement, for use in nuclear reactors having a preferably long lifeunder such a high level radiation environment as is typical in a reactorcore, and methods for repairing the same.

The present invention relates to a precipitation hardening type singlecrystal austenitic steel having a preferable resistance to stresscorrosion cracking and to neutron irradiation composed of a parent phasewith a chemical composition of:

Si: at most 1% by weight,

Mn: at most 2% by weight,

Ni: 9˜21% by weight,

Cr: 14˜20% by weight,

unavoidable impurities in manufacturing of the steel: at most 0.5% byweight, and

the balance being Fe, characterized by a parent phase that is composedof an austenitic phase including 0˜10% by volume of ferritic phase atroom temperature.

The austenitic phase has crystallinity of a single crystal, withcomposite carbides containing carbon elements in a range of 0.03˜0.20%by weight, a part of the parent phase elements, and at least one of theadded elements of Ti, Zr, V, Nb, and Ta is precipitated by a solutionheat treatment and a subsequent aging treatment in the parent phase inorder to enhance the strength of the parent phase. The single crystalaustenitic steel has a yield strength of at least 220 MPa by dispersingthe composite carbides in the parent phase at room temperature.

Furthermore, the present invention relates to a precipitation hardeningtype single crystal austenitic steel with a preferable corrosionresistance composed of a parent phase with a chemical composition of:

at most 1% by weight,

Mn: at most 2% by weight,

Ni: 9˜14% by weight,

Cr: 18˜20% by weight,

unavoidable impurities in manufacturing of the steel: at most 0.5% byweight, and

the balance being Fe, characterized by a parent phase that is composedof an austenitic phase including 0˜10% by volume of a ferritic phase atroom temperature. The austenitic phase has the crystallinity of a singlecrystal, with composite carbides containing a carbon element in a rangeof 0.03˜0.20% by weight, a part of the parent phase elements, and atleast one of the added elements of Ti, Zr, V, Nb, and Ta is precipitatedby a solution heat treatment and a subsequent aging treatment in theparent phase in order to enhance the strength of the parent phase. Thesingle crystal austenitic steel has a yield strength of at least 220 MPaby dispersing the composite carbides in the parent phase at roomtemperature.

Furthermore, the present invention relates to a precipitation hardeningtype single crystal austenitic steel having a preferable corrosionresistance composed of a parent phase with a chemical composition of:

Si: at most 1% by weight,

Mn: at most 2% by weight,

Mo: 1.0˜4.0% by weight,

Ni: 10˜15% by weight,

Cr: 14˜18% by weight,

unavoidable impurities in manufacturing of the steel: at most 0.5% byweight, and

the balance being Fe, characterized by a parent phase that is composedof an austenitic phase including 0˜10% by volume of ferritic phase atroom temperature. The austenitic phase has the crystallinity of a singlecrystal, with composite carbides containing a carbon element in a rangeof 0.03˜0.20% by weight, a part of the parent phase elements, and atleast one of the added elements of Ti, Zr, V, Nb, and Ta is precipitatedby a solution heat treatment and a subsequent aging treatment in theparent phase in order to enhance the strength of the parent phase. Thesingle crystal austenitic steel has a yield strength of at least 220 MPaby dispersing the composite carbides in the parent phase at roomtemperature.

The above described three kinds of precipitation hardening type singlecrystal austenitic steels relating to the present invention can containPd as a chemical composition. The amount of Pd contained in the steel ispreferably in a range of 0.5˜2.0% by weight.

The present invention relates to a manufacturing method for a singlecrystal of austenitic Cr--Ni type stainless steel reinforced withprecipitated fine carbides. The manufacturing method comprises the stepsof melting austenitic Cr--Ni type stainless steel containing carbon andat least one of Ti, Zr, V, Nb, and Ta for generating carbide of themetals, then solidifying the molten austenitic base material in onedirection to obtain a single crystal, thermally treating the generatedcarbides for re-decomposing Cr carbide and the above carbide of metalsgenerated in the course of cooling the single crystal in two steps (twosteps decomposing heat treatment), the first step is at a temperature ina range of 1050°˜1150° C. for carbides having a low-melting point, andthe second step is at a temperature in a range of 1200°˜1300° C. forcarbides having a high melting point, subsequently cooling the singlecrystal rapidly, and again thermally treating the single crystal in asolid solution state for precipitating fine carbides in the austeniticsingle crystal base material (precipitation heat treatment).

The precipitated carbides occupy 5˜20% in area of the material, and areat most 0.5 μm in diameter, preferably most of the carbides are within0.02˜0.3 μm in diameter, and at most 1 μm in length, preferably most ofthe carbides are within 0.1˜0.8 μm in diameter. Crystallized carbidesare at most 0.1 mm in width (preferably most of the carbides are withina range of 0.02˜0.08 mm) and at most 0.5 mm in length, preferably mostof the carbides are within a range of 0.02˜0.25 mm.

The austenitic stainless steel having a single crystal austenitic phase,generated a by directional solidification relating to the presentinvention, has a yield strength of 200˜400 MPa, a tensile strength of500˜700 MPa, an elongation percentage of 25˜50%, and a contractionpercentage of 35˜60% at room temperature.

The present invention relates to members for a nuclear reactor core,which are exposed to pure water at a high temperature and a highpressure and are irradiated with neutrons from nuclear fuel rods. Themembers are composed of precipitation hardening type single crystalaustenitic steel having the above described chemical composition. Theprecipitation hardening type single crystal austenitic steel is composedof an austenitic phase including 0˜10% by volume of ferritic phase, andthe austenitic phase has the crystallinity of a single crystal,composite carbides containing carbon elements in a range of 0.03˜0.20%by weight, a part of the parent phase elements, and at least one of theadded elements of Ti, Zr, V, Nb, and Ta precipitated by a solution heattreatment and a subsequent aging treatment in the parent phase in orderto enhance the strength of the parent phase. The single crystalaustenitic steel has a yield strength of at least 220 MPa by dispersingthe composite carbides in the parent phase at a room temperature.

The present invention relates to members which are composed of aprecipitation hardening type single crystal austenitic steel having theabove described chemical composition. The precipitation hardening typesingle crystal austenitic steel is composed of an austenitic phaseincluding 0˜10% by volume of ferritic phase, and the austenitic phasehas the crystallinity of a single crystal, composite carbides containingcarbon elements in a range of 0.03˜0.20% by weight, a part of the parentphase elements, and at least one of the added elements of Ti, Zr, V, Nb,and Ta, precipitated by a solution heat treatment and a subsequent agingtreatment in the parent phase in order to enhance the strength of theparent phase. The single crystal austenitic steel has a yield strengthof at least 220 MPa by dispersing the composite carbides in the parentphase at room temperature.

The present invention relates to fastening members which are composed ofa precipitation hardening type single crystal austenitic steel, havingthe above described chemical composition. The precipitation hardeningtype single crystal austenitic steel is composed of an austenitic phaseincluding 0˜10% by volume of ferritic phase, and the austenitic phasehas the crystallinity of a single crystal, composite carbides containingcarbon elements in a range of 0.03˜0.20% by weight, a part of the parentphase elements, and at least one of the added elements of Ti, Zr, V, Nb,and Ta, precipitated by a solution heat treatment and a subsequent agingtreatment in the parent phase in order to enhance the strength of theparent phase. The single crystal austenitic steel has a yield strengthof at least 220 MPa by dispersing the composite carbides in the parentphase at room temperature.

The present invention relates to nuclear reactor cores provided withupper grid plates and core support plates, wherein fastening memberswhich are used for assembling the upper grid plates and the core supportplates, fuel support brackets, and peripheral fuel support brackets, arecomposed of a precipitation hardening type single crystal austeniticsteel with the above described chemical composition. The precipitationhardening type single crystal austenitic steel is composed of anaustenitic phase including 0˜10% by volume of ferritic phase, and theaustenitic phase has the crystallinity of a single crystal, compositecarbides containing carbon element in a range of 0.03˜0.20% by weight, apart of the parent phase elements, and at least one of the addedelements of Ti, Zr, V, Nb, and Ta, precipitated by a solution heattreatment and a subsequent aging treatment in the parent phase in orderto enhance the strength of the parent phase. The single crystalaustenitic steel has a yield strength of at least 220 MPa by dispersingthe composite carbides in the parent phase at room temperature.

The present invention relates to nuclear reactor cores provided withshrouds encircling nuclear fuel assemblies, wherein fastening memberswhich are used for fixing the shrouds are composed of a precipitationhardening type single crystal austenitic steel with the above describedchemical composition. The precipitation hardening type single crystalaustenitic steel is composed of an austenitic phase including 0˜10% byvolume of ferritic phase, and the austenitic phase has the crystallinityof a single crystal, composite carbides containing carbon elements in arange of 0.03˜0.20% by weight, a part of the parent phase elements, andat least one of the added elements of Ti, Zr, V, Nb, and Ta,precipitated by a solution heat treatment and a subsequent agingtreatment in the parent phase in order to enhance the strength of theparent phase. The single crystal austenitic steel has a yield strengthof at least 220 MPa by dispersing the composite carbides in the parentphase at room temperature.

The present invention relates to a bolt-fastening repairing the methodfor repairing internal structures and apparatus of nuclear reactor coreshaving damaged portions which are inadequate for the integrity of thenuclear reactor. The bolts which are used for the bolt-fastening jigs inthe repairing method for preventing ageing growth of the inadequatedamaged portions and to maintain the strength of the peripheral portionsof the reactor core, including the inadequate damaged portions, arecomposed of a precipitation hardening type single crystal austeniticsteel with the above described chemical composition. The precipitationhardening type single crystal austenitic steel is composed of anaustenitic phase including 0˜10% by volume of ferritic phase, and theaustenitic phase has the crystallinity of a single crystal, compositecarbides containing carbon elements in a range of 0.03˜0.20% by weight,a part of the parent phase elements, and at least one of the addedelements of Ti, Zr, V, Nb, and Ta, precipitated by a solution heattreatment and a subsequent aging treatment in the parent phase in orderto enhance the strength of the parent phase. The single crystalaustenitic steel has a yield strength of at least 220 MPa by dispersingthe composite carbides in the parent phase at room temperature.

The present invention aims at preventing grain boundary stress corrosioncracking of austenitic stainless steel, which is generated wherever agrain boundary exists, by grain boundary corrosion sensitization causedby the influence of welding heat under a non-irradiated condition, or byirradiation induced sensitization under a neutron irradiated condition.However, stress corrosion cracking is not generated in a structurewherein any grain boundaries connecting heterogeneous phases to eachother do not exist, for instance, ferritic phases dispersed in theaustenitic single phase. Therefore, a single crystal steel having anaustenitic single phase can be an objective of investigation. A singlecrystal having the same parent phase structure as a polycrystallinecrystal decreases in strength because of lack of grain boundaries, andthe single crystal can not achieve its role when it is used, forinstance, as a fastening member. The problem to be solved by the presentinvention is to improve the strength of the single crystal steel. Inthis regard, the strength of the single crystal steel can be improved byprecipitating carbon and additive metals, which have previously beencontained in the steel in a solid solution condition, in an adequatedistributing state with a view toward improving the strength by acarbide precipitating reaction in a heat treatment after singlecrystallization. Furthermore, the precipitated carbide phase in adispersion state has a function of quenching the point of the defect.The defect is generated by recoiling atoms under irradiation and thenmoves among lattices. Ageing embrittlement of the parent material can beprevented by avoiding accumulation of the defects.

As explained hereinafter, the inventors of the present inventionevaluated carbide precipitated single crystal austenitic stainless steelby performing stress corrosion cracking tests under a high temperatureand a high pressure water environment which simulates a corrosionenvironment in a nuclear reactor, as well as tensile strength tests, andsimulated irradiation tests equivalent to neutron irradiation damage ina nuclear reactor core for 20 years operation. It was found that thecarbide precipitated single crystal austenitic stainless steel has apreferable stress corrosion cracking resistance, an enhanced strength,and a superior irradiation resistance.

The single crystal having no successive grain boundaries can be preparedby the steps of melting austenitic stainless steel with an adjustedchemical composition, then directionally solidifying the moltenstainless steel gradually, selecting one of the crystal grains whichgrow from the initial crystal growing plane, and growing crystal grainsaligned in a direction along the selected crystal orientation.

The melting of the austenitic stainless steel is preferably performed atleast at 1500° C. for ease in melting and at most at 1659° C. forcontrolling the reaction of the stainless steel with the casting die. Inorder to avoid contamination of the molten steel with gaseous elementsin air, atmosphere around the molten steel is maintained at a reducedpressure of less than 3×10⁻³ Torr or in an inert gas atmosphere, such asargon gas. The condition for solidifying the molten steel in onedirection is such that the same atmosphere as that of melting is usedfor the same reason as stated above, and the solidifying speed is in arange of 1˜50 cm/h. A solidifying speed of more than 50 cm/h makes itdifficult to prepare a single crystal ingot for large size members, anda solidifying speed of less than 1 cm/h causes a defect due to areaction of the molten steel with the casting die. Therefore, asolidifying speed equal to or more than 1 cm/h is preferable.

The single crystal, after the directional solidification, containsmetallic carbides which have been precipitated and coagulated to formcoarse grains during the solidification. However, the coarse grains donot have any advantageous effect in improving the strength. Therefore, asolution heat treatment for decomposing and making the coarse carbides asolid solution becomes necessary. The solution heat treatment preferablycomprises the steps of annealing the carbide at a temperature in a rangeof 1050°˜1150° C. below its melting point for 1˜2 hours to give thecarbide a low melting point, mainly Cr carbide, a solid solution, andsubsequently annealing the carbide at a temperature in a range of1200°˜1300° C. for 2˜5 hours to decompose residual complex carbides ofCr, Mo, Si, Ti, Zr, V, Nb, Ta and the like having a high melting pointto make a solid solution. If the temperature during the solution heattreatment is higher than the melting point of the carbides, a void whichcan be seen in the structure of the solidified steel is generated. Forprecipitating carbides as hardening particles, an ageing treatment at atemperature in a range of 600°˜900° C. for more than 16 hours andpractically less than 30 hours after rapid cooling from the previousannealing at a higher temperature is preferable.

In accordance with the above ageing treatment, a yield strength of atleast 220 MPa can be obtained by precipitating carbides with an averageparticle diameter smaller than submicrons with a number density of morethan 10¹⁴ /cm³ or more than 200×10⁻² /μm². The number density of theprecipitated carbides is desirably 250˜800×10⁻² /μm², and preferably270˜540×10⁻² /μm². The precipitated fine carbides preferably have anarea fraction of 5˜20%, at most 0.5 μm in diameter, and at most 1 μm inlength. When a ratio of the amount of Nb and the amount of C, Nb/C, isless than 10, a ferritic phase is generated of at most 10%, andprecipitated carbides of Nb, or Nb and Mo, of at most 1 mm wide and 0.5mm long exist. The precipitated carbides are more preferable when theyare as small as possible.

As explained above, irrespective of its generating mechanism, stresscorrosion cracking starting from grain boundaries can be prevented byeliminating grain boundaries in the material. A single crystal with nograin boundaries causes a decrease in mechanical strength, but thisdecrease can be compensated by precipitating carbides with the solutionheat treatment. The present invention aims at providing austeniticstainless steel with a high strength characteristics, by whichgeneration of cracking is substantially prevented.

At least 14% of Cr content is necessary for enhancing the corrosionresistance, for forming the austenitic phase, and for preparing a largesize single crystal material from the austenitic phase material.However, the addition of a large amount of Cr exceeding 20% causesembrittlement of the material because an s-phase is formed in thesolidifying process of the single crystal.

Especially, 16˜20% of Cr content, within a range of the stableaustenitic phase, depending on the Ni content is desirable inconsideration of a decrease of the Cr content, in the parent phase, inaccordance with generation of chromium carbides and stabilization of theaustenitic phase. Especially, a range of 17˜19% is preferable.

At least 9% Ni is contained in the material to stabilize the austeniticphase and enhance corrosion resistance. An addition of a large amount ofNi to the material increases corrosion when the material is used, forinstance, in a nuclear reactor by causing an electrochemical reaction atcontact points with other members depending on the difference in thechemical composition under a same corrosion environment. Inconsideration of SUS 304, SUS 316 and their L material which arefrequently used in a nuclear reactor, the Ni content of 9˜21% isdesirable in connection with the Cr content. Especially, 12˜20.5% ispreferable. Further, 12˜14%, or 19˜21% is more preferable.

Si and Mn are added to the material as deoxidizing agents, and Mn isadded further as a desulfurizing agent. A Si content of at most 1 %, anda Mn content of at most 2% in the material is desirable in accordancewith commercially available SUS 304, SUS 316. Especially, 0.05˜0.35% forSi is preferable, and further, 0.05˜0.15%, or 0.2˜0.4% is morepreferable. Especially, 0.01˜1.5% for Mn is preferable, and further,0.01˜0.1%, or 0.5˜1.5% is more preferable.

Mo is an essential element for increasing the corrosion resistance andenhancing the solid solution. It is necessary to add at least 1% Mo.However, the addition of Mo at more than 4% is not necessary because Mocan be a cause of forming an s-phase. Furthermore, Mo contributes togeneration of complex carbides.

C is an essential element for forming metallic carbides. In order toprepare Cr carbide and at least one of the carbides among carbides ofTi, Zr, V, Nb, and Ta, for obtaining a yield strength of at least 220MPa, at least 0.03% of the C content is desirable for the lower limit,and 0.2% of the C content is desirable for the upper limit. The contentof 0.03% by weight equals approximately 0.4% or less in atomic weight,and 0.2% by weight equals approximately 1% in atomic weight. When the Ccontent is less than 0.03%, most of the C is consumed in forming Crcarbides which readily become relatively coarse grain in aging, and thegenerated amount of other metal carbides decreases. When the C contentis more than 0.2%, decreasing strength and elongation cause problemsbecause coarse excess carbides are generated and grown. Especially, arange of 0.10˜0.15% C is desirable.

Ti, Zr, V, Nb, and Ta are elements highly capable of generatingcarbides, and are necessary as enhanced precipitating agents in thepresent invention. The energy necessary for formation is lower inaccordance with the above described order. The amount of elements to beadded depends on the kind of at least one of the elements selected fromthe above group and the carbon content. For instance, if only one of theabove five elements is added, an addition of the amount by weight forrespective elements, Ti, Zr, V, Nb, and Ta, of approximately Ti:4×(C %),Zr:7.5×(C %), V:4×(C %), Nb:7.5×(C %), and Ta:13×(C %) is preferable.That means, 0.12˜0.8% for Ti, 0.22˜1.5% for Zr, 0.12˜0.8% for V,0.22˜1.5% for Nb, and 0.39˜2.6% for Ta is respectively preferable.Because the above respective elements form carbides with carbon in aratio of one to one, the amount of the elements to be added is desirablysomewhat less than an equivalent value to the C content in considerationof the consumption of carbon elements for forming Cr carbide.

Further, the preferable content of the elements can be adequatelydesignated in atomic percentage (%) as1<{Ti(%)+Zr(%)+V(%)+Nb(%)+Ta(%)}/{C(%)+0.043}<2.

Pd is necessary for adding high corrosion resistance to the parentmaterial containing Cr and Ni to enhance its function. In order to beeffective, Pd must be contained to at least 0.5%, and the upper limit is2.0%. The addition of more than 2.0% Pd influences the austenitic phasestructure, and the corrosion resistance is increased by adding Pd morethan 2.0%, which enhances corrosion of other members in the same manneras the excess adding of Ni. Therefore, a range of 0.4˜1.5% is especiallypreferable.

The austenitic phase is a stable structure in the environment, and isnecessary for obtaining a large size single crystal. The ferritic phasecan exist at most at 10% in the austenitic phase, but a whole austeniticphase structure is desirable.

Plastic working at most at 5%, which is within a range of keeping asingle crystal structure, can be added to the steel of the presentinvention. Further, the plastic working is applicable when therespective heat treatments can be more effective by adding the plasticworking before the solution heat treatment and/or the aging heattreatment.

The steel of the present invention has a yield strength of 200˜400 MPa,a tensile strength of 500˜700 MPa, an elongation percentage of 25˜50%,and a contraction percentage of 35˜60% at room temperature. Especially,the steel of the present invention has characteristics of yield strengthof 250˜450 MPa, tensile strength of 550˜650 MPa, elongation percentageof 30˜42%, and contraction percentage of 35˜56% at room temperature.

The steel of the present invention is used not only in nuclear reactorcores, but also in a water-cooled environment or a hydrogen-existingenvironment, similar to structural materials which receiveradiation-irradiated damage, especially for bracket cooling tubes for anuclear fusion reactor first wall and shell. Generally, the steel isused for strong members which are used under an environment whereingrain boundaries mainly cause deterioration of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical cross section indicating a composition ofa manufacturing apparatus for a precipitation hardening type austeniticstainless steel, which is one of embodiments of the present invention;

FIG. 2 is a graph indicating a variation of Vickers hardness dependingon aging temperatures of the steel species No. 7, which is one of theembodiments of the present invention;

FIG. 3 is a perspective drawing indicating a testing method for stresscorrosion cracking;

FIG. 4 is a perspective partial cross sectional view of a nuclearreactor core indicating an embodiment using the austenitic steel of thepresent invention;

FIG. 5 is a schematic partial plan view of an upper grid plate of thenuclear reactor core;

FIG. 6 is a vertical cross sectional view taken along the line VI--VI ofFIG. 5;

FIG. 7 is an enlarged detail view taken from the VII portion of FIG. 6;

FIG. 8 is an enlarged partial plan view of the upper grid plate of thenuclear reactor core;

FIG. 9 is an enlarged detail view taken from the IX portion of FIG. 8;

FIG. 10 is an enlarged detail view taken from the X portion of FIG. 8;

FIG. 11 is a perspective view of a core supporting plate in the nuclearreactor core;

FIG. 12 is a perspective view of a fuel supporting bracket in thenuclear reactor core;

FIG. 13 is a cross sectional view of a peripheral fuel supportingbracket in the nuclear reactor core;

FIG. 14 is an enlarged detail view taken from the XIV portion of FIG.11;

FIG. 15 is a perspective partial view of a shroud indicating a repairingmethod for a defected portion of the shroud;

FIG. 16 is a cross sectional view of a portion repaired with a taperlessbolt which is an embodiment of the present invention; and

FIG. 17 is a cross sectional view of a portion repaired with a taperedbolt, which is an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment 1

Referring to FIG. 1, an embodiment of the method of manufacturing aprecipitation hardening type single crystal austenitic steel relating tothe present invention will be explained hereinafter.

The apparatus indicated in FIG. 1 comprises a high frequency meltingfurnace 2 for preparing molten metal 1, a casting die 3, a body 4, aselector 5, a starter 6 which is placed on a water-cooled chill 9, andcasting die super heater 8 provided with a casting inlet 7 covering theabove casting die 3, the body 4, the selector 5, and the starter 6. Thecasting die 3 was fixed on the water-cooled chill 9, the casting die 3was super-heated to 1550° C. by the casting die super heater 8, andaustenitic stainless steel containing carbon and carbides, generatingmetals shown in Table 1, was melted to molten metal 1 by the highfrequency melting furnace 2 and cast in the casting die 3 through thecasting inlet 7.

In Table 1, a result of analysis of single crystals of 12 steel specieswhich were treated with final heat treatment according to the presentinvention is shown.

The casting temperature was 1550° C., the body 4 was maintained at 1150°C. for 5 minutes after the casting, subsequently the water-cooled chill9 was moved downward in the direction of arrow 10, and finally thecasting die 3 was withdrawn from the casting die super heater 8 so thatthe molten metal 1 in the body 4 was solidified in one direction fromthe water-cooled chill 9 side. At that time, the casting die superheater 8 was maintained at 1550° C. until the solidification in onedirection was finished. The withdrawing velocity of the casting die waskept at a constant rate, 15 cm/h, and the molten metal in the body 4 wasmaintained in an atmosphere of reduced pressure, 8×10⁻³ Torr. After thecasting, many crystals were generated in the starter 6. However, as thecasting die 3 was withdrawn downward, solidification of the crystalsproceeded gradually, of the crystals only one crystal was selectedduring solidifying in the selector 5, and a single crystal was obtainedfrom the molten metal in the body 4.

                                      TABLE 1                                     __________________________________________________________________________        Chemical composition (% by weight)                                        Steel                                                                             Balance in all species is Fe.                                             sp. No.                                                                           C  Si Mn P  S  Ni Cr Mo Pd Ti Zr V  Nb  Ta                                __________________________________________________________________________    1   0.114                                                                            0.085                                                                            0.026                                                                            0.032                                                                            0.017                                                                            20.41                                                                            17.42                                                                            -- -- -- -- --  0.925                                                                            --                                2   0.152                                                                            0.091                                                                            0.019                                                                            0.032                                                                            0.021                                                                            20.12                                                                            17.27                                                                            -- 0.943                                                                            0.615                                                                            -- -- --  --                                3   0.149                                                                            0.33                                                                             1.28                                                                             0.030                                                                            0.019                                                                            12.06                                                                            18.52                                                                            -- -- -- -- -- 1.08                                                                              --                                4   0.151                                                                            0.28                                                                             1.21                                                                             0.032                                                                            0.022                                                                            12.32                                                                            18.14                                                                            -- 1.260                                                                            0.639                                                                            -- -- --  --                                5   0.136                                                                            0.36                                                                             1.21                                                                             0.030                                                                            0.016                                                                            12.18                                                                            18.62                                                                            -- -- -- -- 0.581                                                                            --  --                                6   0.119                                                                            0.32                                                                             1.19                                                                             0.033                                                                            0.020                                                                            12.21                                                                            18.13                                                                            -- 0.476                                                                            -- -- -- --  --                                7   0.144                                                                            0.31                                                                             1.26                                                                             0.031                                                                            0.022                                                                            13.35                                                                            17.02                                                                            2.61                                                                             -- -- -- -- 1.12                                                                              --                                8   0.182                                                                            0.33                                                                             1.22                                                                             0.028                                                                            0.021                                                                            12.66                                                                            17.32                                                                            2.73                                                                             0.643                                                                            0.773                                                                            -- -- --  --                                9   0.154                                                                            0.38                                                                             1.28                                                                             0.032                                                                            0.019                                                                            13.05                                                                            17.39                                                                            2.31                                                                             -- 0.433                                                                            -- -- --  2.23                              10  0.141                                                                            0.28                                                                             1.28                                                                             0.032                                                                            0.022                                                                            12.66                                                                            17.51                                                                            2.48                                                                             -- -- 0.972                                                                            -- --  --                                11  0.137                                                                            0.31                                                                             1.28                                                                             0.032                                                                            0.023                                                                            12.66                                                                            17.67                                                                            2.83                                                                             0.722                                                                            -- -- -- 1.06                                                                              --                                12  0.161                                                                            0.27                                                                             1.28                                                                             0.032                                                                            0.018                                                                            12.66                                                                            17.92                                                                            2.13                                                                             -- 0.24                                                                             0.327                                                                            -- --  0.59                              __________________________________________________________________________

In accordance with the above method, a single crystal of 20 mm indiameter and 20 cm long, and a single crystal of 15 mm thick, 70 mmwide, and 120 cm long were obtained depending on the capacity of thecasting die 3. The existence of the single crystal was confirmed by amicro-etching technique. Casting products wherein an austenitic phasewas composed of a single crystal could be prepared from all of the steelspecies shown in Table 1.

At the time of preparation, even if the casting temperature wasdesignated as 1650° C., there was no problem for obtaining a singlecrystal. If the atmosphere was replaced with inert argon gas, there wasno influence on preparation of the single crystal. Even if the castingdie withdrawing velocity was varied to 1 cm/h or 50 cm/h in cases of thesteel species No. 17 and 10, single crystals were obtained as well.

However, coarsely crystallized carbides or precipitated phases otherthan the carbides composed of Ti, Zr, V, Nb, and Ta, both of which areassumed to be formed at the time of solidification, are formed generallyas a network in all of the steel species. For instance, the steelspecies No. 7 which contains Nb forms coarse carbides as a network. Thecarbides hardly contribute to the mechanical strength of the steel asdescribed later.

Next, a fining process of the steel species by solution heat treatmentwill be explained. In order to decompose the coarse carbides andprecipitated phases and to make them a solid solution, a solution heattreatment at a temperature range of 1200°˜1250° C. was performed for2˜24 hours. The microstructure indicated that all of the steel speciesshown in Table 1 had undecomposed carbides in response to a solutionheat treatment for 2˜5 hours, and voids were observed in the parentphase. The undecomposed carbides disappeared in response to a solutionheat treatment for more than 6 hours, but the void density wasincreased. Generation of voids is undesirable in view of the mechanicalstrength of the material. It is considered that generation of voids isdue to the carbides or the precipitated phases composed of mainly Crhaving a low melting point which are previously formed in thesolidification melt in the annealing process, and which remain as voidsafter cooling. Accordingly, a first solution heat treatment at atemperature in a range of 1050°˜1150° C. was performed for 60 hours inorder to decompose the carbides or the precipitated phases having a lowmelting point completely, and subsequently, a second solution heattreatment at a temperature in a range of 1200°˜1300° C. was performedfor 2˜5 hours in order to decompose the carbides having a high meltingpoint and to make them a solid solution. Substantially, the abovesolution heat treatment condition varied depending on respective steelspecies as shown in Table 2 for making residual carbides or theprecipitated phases 1% or less in a fraction of the cross sectionaloccupying area.

An ageing treatment for precipitation from the solid solution conditionwas performed on all the steel species at a temperature of every 50° C.in a range of 600°˜900° C. for 16 hours. In order to find an ageingcondition which gives the maximum mechanical strength, a hardness testwas performed on samples which were treated by the ageing treatment atvarious temperature. A relationship between the ageing temperatures andVickers hardness of the steel species No. 7 is shown in FIG. 2. It wasconfirmed that the sample treated with ageing at 800° C. which showedthe maximum hardness had rod shaped complex carbides which had a crystalorientation, a submicron size, and a distribution with submicronintervals by 330˜470×10⁻² pieces/μm², and the precipitated carbides hada fraction of cross sectional occupying area of about 12%. Analysis witha EPMA and a TEM revealed that the complex carbides were composed of Nb,Cr, Mo, Si, and the like. The precipitated carbides were at most 0.5 μmin diameter, mostly 0.02˜0.3 μm, and at most 1 μm long, mostly 0.1˜0.8μm. The crystallized carbides had a fraction of cross sectionaloccupying area of about 6%, and were at most 0.1 mm wide, mostly0.01˜0.08 mm, and at most 0.5 mm long, mostly less than 0.02˜0.25 mm.

The solution heat treatment shown in Table 2 and the ageing heattreatment obtaining the maximum hardness were performed on all of the 12steel species, samples for tensile test of parallel portion 16 mm longand 5 mm in diameter were prepared, and tensile tests were performedwith nominal strain velocity of 2.1×10⁻⁴ (/second). The results of thetensile tests are shown in Table 3. In the Table 3, the steel speciesnumbers attached with the * mark such as No.1*, 4*, and 7* are theresults on single crystal steels without any ageing treatment.

                  TABLE 2                                                         ______________________________________                                                 First solution heat                                                                           Second solution heat                                 Steel    treatment       treatment                                            species  Tempera-            Tempera-                                         No.      ture (°C.)                                                                      Hours (h)  ture (°C.)                                                                    Hours (h)                                 ______________________________________                                        1        1150     60         1250   3                                         2        1150     60         1250   3                                         3        1100     60         1250   3                                         4        1150     60         1250   3                                         5        1150     60         1270   5                                         6        1100     60         1250   3                                         7        1150     60         1250   3                                         8        1150     60         1250   3                                         9        1150     60         1270   5                                         10       1150     60         1270   5                                         11       1100     60         1250   3                                         12       1150     60         1270   5                                         ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Steel    0.2% Yield                                                                              Tensile                                                    species  strength  strength  Elonga-                                                                             Contrac-                                   No.      (MPa)     (MPa)     tion (%)                                                                            tion (%)                                   ______________________________________                                         1       306       582       39.6  51.0                                        2       337       617       34.3  56.0                                        3       291       587       37.2  49.0                                        4       283       586       30.9  37.5                                        5       322       595       31.7  46.0                                        6       264       560       37.0  49.0                                        7       292       594       37.5  48.0                                        8       306       581       34.0  46.0                                        9       274       562       39.1  50.5                                       10       318       621       33.4  51.0                                       11       256       571       40.8  42.0                                       12       349       618       32.7  48.5                                       1*       184       191       80.5  38.0                                       4*       170       178       62.0  56.0                                       7*       162       166       58.5  49.0                                       ______________________________________                                    

Test pieces of 2 mm thick, 10 mm wide, and 50 mm long were prepared froma carbide dispersed single crystal austenitic steel of 15 mm thick, 70mm wide, and 120 mm long, which was prepared by the above describedprocess, and a CBB test which was effective for stress corrosioncracking was performed with commercially available SUS 316 and SUS 304steel. Previously, sensitizing heat treatment was performed on the abovedescribed test pieces at 620° C. for 24 hours. FIG. 3 is a perspectiveview indicating a method for the CBB test. The test piece 11 was held inan span of the holder 13 with graphite fiber wool 12, which provided agap to the test piece 11, bolts were inserted into the holes 14respectively, the test piece was fastened tightly in the span of theholder 13 with a curvature by the bolts, and the holder 13 with the testpiece 11 was placed in an autoclave for a stress corrosion crackingtest. The test condition was such that the test piece was immersed intowater of high temperature, 288° C., and high pressure, 85 kg/cm²(dissolved amount of oxygen 8 ppm), for 500 hours. Subsequently, thetest piece 11 was taken out from the autoclave, and generation of crackswas determined by observation on the cross section of the test piece 11.Many cracks of 1˜2 mm deep were observed on the SUS 316 and SUS 304steel test pieces. On the contrary, no cracks were observed on allsingle crystal steel relating to the present invention, and highcorrosion resistance of the single crystal steel relating to the presentinvention was confirmed.

Embodiment 2

After the steel species No. 7 of austenitic single crystal stainlesssteel prepared by the embodiment 1 of the present invention was heattreated under the above described condition, various structural membersof a boiling water nuclear reactor shown in FIG. 4 were manufacturedwith the austenitic single crystal stainless steel. The nuclear water isoperated with steam of a temperature at 286° C. and a pressure of 70.7atg, is able to generate electric power of 500, 800, 100 MW as anoutput. The respective members of the reactor are as follows: 51 . . .poison curtain, 52 . . . core support plate, 53 . . . neutron detectinginstrument tube, 54 . . . control rod, 55 . . . core shroud, 56 . . .upper grid plate, 57 . . . fuel assembly, 58 . . . upper end plate spraynozzle, 59 . . . bent nozzle, 60 . . . pressure vessel lid, 61 . . .flange, 62 . . . instrumentation nozzle, 63 . . . water/steam separator,64 . . . shroud head, 65 . . . feed water inlet nozzle, 66 . . . jetpump, 68 . . . steam dryer, 69 . . . steam outlet nozzle, 70 . . . feedwater sparger, 71 . . . core spray nozzle, 72 . . . lower core grid, 73. . . circulating water inlet nozzle, 74 . . . baffle plate, 75 . . .control rod guide tube.

The upper grid plate 56 comprises a rim body 21, flanges 22, and gridplates 35 which are made of a polycrystalline rolled material of SUS 316steel. The grid plates 35 only intersect each other and are not fixedeach other. The core support plate 52 is made of a polycrystallinerolled material of SUS 316 steel, manufactured from a sheet of rolledplate, in which holes for fixing fuel support brackets are provided, andthe plate 52 is fixed to the reactor vessel at its peripheral portion.Accordingly, the above described members have no welding portions at acentral portion which receives neutron irradiation.

FIG. 5 is a partial plan view of the upper grid plate. FIG. 6 is a crosssectional view taken along the line VI--VI in FIG. 5, and FIG. 7 is apartial enlarged cross sectional view of the VI portion in FIG. 6. Theabove described alloy relating to the present invention was applied tothe bolt 23 shown in FIG. 7. The bolt 23 of the present invention wasused for fixing the rim body 21 and the upper flange 22, and the screwwas manufactured by cutting a rod shaped material.

FIG. 8 is a partial enlarged view of the upper grid plate, FIG. 9 is apartial enlarged view of the IX portion in FIG. 8, and FIG. 10 is apartial enlarged view of the X portion in FIG. 8. Bolts and nuts forfastening and fixing the grid plate 31 and the support plate 32 of theupper grid plate 56, and the bolts 36 and the nuts 37 for fastening thegrid plate 31 and the support plate 32, and the support plate 32 and thegrid plate 35 were manufactured from a single crystal as well as theabove.

FIG. 11 is a schematic cross sectional view of a core supporting plate52, and the core supporting plate was provided with fuel supportingbrackets shown in FIG. 12, peripheral fuel supporting brackets shown inFIG. 13, eye bolts 42 and washers 43 shown in FIG. 14, and the like.FIG. 14 is a partial enlarged view of the XIV portion in FIG. 11. Theabove described fuel supporting bracket, peripheral fuel supportingbracket, eye bolt 42, and washer 43 as shown in FIGS. 12˜14 wereprepared from the above described single crystal austenitic steel of thepresent invention.

The above described members, manufactured by the method of the presentinvention, were irradiated with neutrons by 1×10²² n/cm² (>1 MeV) undera condition simulated in a boiling water nuclear reactor. As a result,no irradiation induced stress corrosion cracking was observed on anybolts or nuts. In accordance with this result, the upper grid plate andthe core support plate are assumed to be capable of operating for 40years without need for replacing. Especially, it is important to composemembers, which are used at portions which receive neutron irradiation asmuch as 2×10²² n/cm² (>1 MeV) and high stresses, as the bolts and nuts,and of which represent a surface invisible directly from the outside,with material having high stress corrosion cracking resistance.

Embodiment 3

Next, an embodiment concerning repairing in accordance with the presentinvention will be explained hereinafter.

FIG. 15 indicates an example of the repairing state of a structuralmaterial for a boiling water nuclear reactor core. As a damaged portionwas generated in a core shroud 55 by stress corrosion cracking, a clamp77 in the form of a reinforcing plate for protecting and reinforcingmechanically, was fixed to the core shroud 55 for repairing by therepairing bolts 78 and the repairing nuts 79. In this case, the clamp77, the repairing bolts 78, and the repairing nuts 79 were made of anaustenitic stainless steel single crystal of the present invention.

The tie of the core shroud 55 and the clamp 77 was fastened with ataperless bolt 80 and the repairing nut 79 as shown in FIG. 16. In othercases, the tie of the core shroud 55 and the clamp 77 was fastened witha tapered bolt 81, the repairing nut 79, and a sleeve with slits 82, asshown in FIG. 17. In this case, the clamp 77, the repairing nuts 79, thetaperless bolt 80, the tapered bolt 81, and the sleeve with slits 82were made of an austenitic stainless steel single crystal of the presentinvention.

In accordance with the present embodiment, life of the boiling waternuclear reactor can be extended by repairing various core structure ofthe nuclear reactor with repairing members having preferable stresscorrosion cracking resistance. Further, as the repairing members have asimilar or approximately the same composition to material of surroundingstructural members, the electric potential in high temperature purewater becomes equal. Fine precipitated carbides exist in the singlecrystal made by the method disclosed in accordance with the presentinvention. As grain boundaries of the carbides and the parent phaseoperate as annihilation points of irradiation defects the same ascrystal grain boundaries, an accumulation of the irradiation defects inthe parent phase can be suppressed, and a large advantage forsuppressing so-called irradiation induced embrittlement, creep underirradiation can be realized.

In the present embodiment, the bolts, the nuts, and the clamp were madeof a single crystal. However, manufacturing the grid plate 35 of theupper grid plate, the core support plate 52, the support plate 32, andneutron detecting instrumentation tubes 53 with the same single materialof the present invention as the bolts and the nuts is also significantlyadvantageous.

In accordance with the present invention, generation of irradiationinduced grain boundary-type stress corrosion cracking in structuralmembers made of austenitic stainless steel, which are used in aradiation irradiated environment, such as a nuclear reactor core, can beprevented. Therefore, the life of structural members in nuclear reactorcores, and the first walls and blanket structural members of nuclearfusion reactors can be extended to make the nuclear reactor operable formore than 40 years, and the safety and reliability of the nuclear fusionreactors can be improved significantly.

What is claimed is:
 1. Precipitation hardening type austenitic stainlesssteel having a stress corrosion cracking resistance which contains:C:0.03˜0.20 & by weight, Si: max. 1% by weight, Mn: max. 2.0% by weight,Ni: 9˜21% by weight, Cr: 14˜20% by weight, and unavoidable impurities:max. 0.5% by weight;wherein said steel comprises an austenitic-ferriticphase including ferritic phases which are at most 10% by volume, or allaustenitic phase, as a parent phase; said austenitic phase is a singlecrystal; and carbides have been substantially formed in an ageing heattreatment and are precipitated in the parent phase.
 2. Precipitationhardening type austenitic stainless steel having a stress corrosioncracking resistance which contains:C: 0.03˜0.20 & by weight, Si: max. 1%by weight, Mn: max. 2.0% by weight, Ni: 9˜21% by weight, Cr: 14˜20% byweight, Pd: 0.5˜2.0% by weight, and unavoidable impurities: max. 0.5% byweight;wherein said steel comprises an austenitic-ferritic phaseincluding ferritic phases which are at most 10% by volume, or allaustenitic phase, as a parent phase; said austenitic phase is a singlecrystal; and carbides have been substantially formed in an ageing heattreatment and are precipitated in the parent phase.
 3. Precipitationhardening type austenitic stainless steel having a stress corrosioncracking resistance which contains:C: 0.03˜0.20 & by weight, Si: max. 1%by weight, Mn: max. 2.0% by weight, Ni: 9˜21% by weight, Cr: 14˜20% byweight, Mo: 1˜4.0% by weight, and unavoidable impurities: max. 0.5% byweight;wherein said steel comprises an austenitic-ferritic phaseincluding ferritic phases which are at most 10% by volume, or allaustenitic phase, as a parent phase; said austenitic phase is a singlecrystal; and carbides have been substantially formed in an ageing heattreatment and are precipitated in the parent phase.
 4. Precipitationhardening type austenitic stainless steel having a stress corrosioncracking resistance which contains:C: 0.03˜0.20 & by weight, Si: max. 1%by weight, Mn: max. 2.0% by weight, Ni: 9˜21% by weight, Cr: 14˜20% byweight, Mo: 1˜4.0% by weight, Pd: 0.5˜2.0% by weight, and unavoidableimpurities: max. 0.5% by weight;wherein said steel comprises anaustenitic-ferritic phase including a ferritic phases which is at most10% by volume, or all austenitic phase, as a parent phase; saidaustenitic phase is a single crystal; and carbides have beensubstantially formed in an ageing heat treatment and are precipitated inthe parent phase.
 5. Precipitation hardening type austenitic stainlesssteel having a stress corrosion cracking resistance which contains:C:0.03˜0.20 & by weight, Si: max. 1% by weight, Mn: max. 2.0% by weight,Ni: 9˜21% by weight, Cr: 14˜20% by weight, at least one element selectedfrom Ti, Zr, V, Nb, and Ta which are defined in atomic percentage as1<{Ti (%)+Zr (%)+V (%)+Nb (%)+Ta (%)}/{C (%)+0.043}<2, and unavoidableimpurities: max. 0.5% by weight;wherein said steel comprises anaustenitic-ferritic phase including ferritic phases which are at most10% by volume, or all austenitic phase, as a parent phase; saidaustenitic phase is a single crystal; and carbides have beensubstantially formed in an ageing heat treatment and are precipitated inthe parent phase.
 6. Precipitation hardening type austenitic stainlesssteel having a stress corrosion cracking resistance which contains:C:0.03˜0.20 & by weight, Si: max. 1% by weight, Mn: max. 2.0% by weight,Ni: 9˜21% by weight, Cr: 14˜20% by weight, at least one element of Mo:1˜4.0% by weight and Pd: 0.5˜2.0% by weight, at least one elementselected from Ti, Zr, V, Nb, and Ta which are defined in atomicpercentage as 1<{Ti (%)+Zr (%)+V (%)+Nb (%)+Ta (%)}/{C (%)+0.043}<2, andunavoidable impurities: max. 0.5% by weight;wherein said steel comprisesan austenitic-ferritic phase including a ferritic phases which is atmost 10% by volume, or all austenitic phase, as a parent phase; saidaustenitic phase is a single crystal; and carbides have beensubstantially formed in an ageing heat treatment and are precipitated inthe parent phase.
 7. Precipitation hardening type austenitic stainlesssteel as claimed in any one of claims 1 to 6, whereinsaid steel containsfine carbides by 5˜20% in a fraction of a cross sectional occupyingarea.
 8. Precipitation hardening type austenitic stainless steel asclaimed in any one of claims 1 to 6, wherein the austenitic steel has ayield strength of at least 220 MPa.
 9. Precipitation hardening typeaustenitic stainless steel as claimed in claim 7, wherein theprecipitated carbides have a maximum diameter of 0.5 μm.